The present invention relates to a method and apparatus for cooling or utilizing hot gas in a reactor in which the lower section of the reactor is provided with a hot gas inlet and a chamber encompassing a fluidized bed, the middle section is provided with a riser, and the upper section with a gas outlet, and the reactor has heat transfer surfaces for recovering heat from solid particles. The invention especially relates to a method, in which hot gas is introduced through the inlet into the lower section of the reactor, and solid particles from the bubbling fluidized bed are fed to the inlet gas for cooling thereof, solid particles are separated from the cooled gas and returned to the fluidized bed, heat is recovered from the separated solid particles, and the cooled gas is discharged through the gas outlet.
Fluid bed reactors are well suited to cooling of hot gases containing molten and/or vaporized components and/or tar-like particles. Gas coolers are suited to. e.g., cooling of exhaust gases from industrial plants and dry purification of gases from partial oxidation of biomass, peat or coal containing dust and tar and other condensing components. The hot gases introduced into the reactor are efficiently cooled by mixing solid particles therewith, such solid particles having been cooled earlier in the reactor.
Finnish patent 64997 teaches cooling of hot gases in circulating fluidized bed reactors. Here hot gases are fed as fluidizing gas into the mixing chamber of the reactor, where the gases cool efficiently as they come into contact with a large volume of solid particles, i.e., bed material. Solid particles are carried by the gas flow through the riser into the upper section of the reactor, where they are separated and then returned to the fluidized bed in the mixing chamber. In the riser, the gas flow conveying solid particles may be cooled by heat transfer surfaces.
A drawback of the method described above is, however, that the hot gases to be cooled have to fluidize a large volume of solid particles, resulting in a high power requirement. On the other hand, a sudden interruption in the power supply may result in the entire bed flowing through the inlet and then out of the reactor.
U.S. Pat. No. 5,205,350 also teaches cooling of hot process gas during stationary fluidization, i.e., a bubbling fluidized bed. Here the hot gas flowing into the reactor is supplied with solid particles as an overflow from the bubbling fluidized bed. The gas and the solid particles entrained therewith flow into a dust collector disposed above the bubbling fluidized bed, from which solid particles then drop back onto the surface of the bubbling fluidized bed as the flow rate of the gas quickly decreases. The bubbling fluidized bed and the gas riser, which is disposed above the dust collector, are provided with heat transfer surfaces.
In the arrangement described above, the particles falling onto the surface of the bubbling fluidized bed are carried along the surface back to the overflow point, where they are immediately taken to recirculation, ending up in the dust collector. Thus, a separate "surface circulation" of hot particles develops above the fluidized bed. These particles do not cool efficiently in the fluidized bed because the particles which are deeper down in the fluidized bed, near the heat transfer surfaces, cannot mix efficiently with the particles present in the "surface circulation".
In the method described above, the riser is considered a natural place for the heat transfer surfaces because the solids and gas flows are swift in the riser. The gas stream, however, causes wear of the heat transfer surfaces in the riser. Wear is partly attributable to the composition of the gas as well as to the dust contained in it, and partly to the high flow rate of the gas.
In some cases, the hot gas flowing to the separator may cause fouling and clogging of the heat transfer surfaces when the gas enters the heat transfer surfaces at too high a temperature. If the hot gas does not cool until it touches the heat transfer surfaces, the impurities will correspondingly condense on or adhere to these surfaces, and not on the circulating mass particles as intended.
Chlorine-containing gases, in particular, cause corrosion at high temperatures and, therefore, superheating of steam to high temperatures is not usually possible in the heat transfer surfaces of the riser. SO.sub.3 may cause problems with the heat transfer surfaces at low temperatures.
According to the present invention an improved method and apparatus, when compared with the above-described methods and apparatus, for cooling or utilizing hot gases in the hot gas treatment of solid material are provided. The method and apparatus of the invention are provided to minimize power consumption and wear of the heat transfer surfaces.
The method and apparatus provide means by which the heat energy released by the hot gas when it cools may be utilized as efficiently as possible, e.g., for generation of superheated steam.
To achieve the above objects, the method of the invention provides for cooling hot gas in a reactor with a bubbling fluidized bed. The cooled gas containing solid particles is conveyed through the riser, which is defined by substantially vertical walls, and introduced into the upper section of the reactor, where solid particles are separated from the gas in a particle separator. The separated solid particles are returned to the bubbling fluidized bed into the outer part thereof, heat is recovered from separated solid particles in a return duct, fluidized bed and/or riser, the solid particles from the inner part of the bubbling fluidized bed are fed into the hot inlet gas.
Exemplary apparatus according to the present invention comprises the following elements: A reactor chamber comprising an upper section and a lower section. A gas inlet tube centrally located in the lower section. A riser extending from the gas inlet tube to the upper section. At least one particle separator located in the upper section for separating particles from gas flowing upwardly in the riser. A bubbling fluidized bed in the lower section, separated into distinct inner and outer substantially concentric parts. A return duct from each of the separators into the outer part of the fluidized bed. A gas outlet from the upper section. Means for cooling particles between the riser and the inner part of the fluidized bed. And, means for introducing particles from the inner part of the fluidized bed into gas introduced into the gas inlet tube to effect cooling of the gas.
According to a preferred embodiment of the invention, the means for feeding cool particles into the hot gas in the inlet may comprise an overflow from the bubbling fluidized bed, directed toward the hot gas flowing through the inlet. Alternatively, the wall between the hot gas inlet and the chamber encompassing the fluidized bed may be provided with openings through which solid particles are introduced into the hot gas flow. Due to a higher static pressure of the fluidized bed, solid material automatically flows through the openings to the hot gas flow, or it may also be conveyed by a transporting gas through the openings into the gas flow. Other mechanisms, such as mechanical devices, may also be provided for this purpose.
In the reactor according to the invention, hot gas is cooled to a substantially lower temperature immediately at the gas inlet by mixing cooled solid particles with the gas, so that the gas cools and the solid particles are correspondingly heated. Besides cooling of gases, the invention may be employed in processes where solid material is heated with hot gases, such as, e.g., heating lime with hot gases. The hot gases are at a temperature over 400.degree. C., typically over 1000.degree.-1300.degree. C. The gas is cooled at least about 100.degree. C., e.g. cooled from over 400.degree. C. to about 200.degree.-400.degree. C.
In a reactor according to the invention, gas may also be cooled in the riser, so that the riser is defined by cooled surfaces, such as for example superheating panels. In the upper section of the reactor, solid particles are separated in a particle separator from the gas which is then exhausted from the reactor. The solid particles are conveyed as a dense suspension, almost as a plug flow, via the return duct back to the bubbling fluidized bed. In the return duct is preferably disposed heat recovery surfaces for recovering the heat energy released by heated solid particles. The heat recovery surfaces are preferably disposed in a dense suspension area.
The return duct is a favorable location for heat transfer surfaces because the particle density is relatively high there, which results in beneficial heat transfer, and because the return duct contains fewer corrosive gaseous components than, e.g., the riser. Hot gas containing molten or condensing components, which might clog the heat transfer surfaces, are also not present in the return duct.
Heat transfer surfaces may also be disposed in the fluidized bed itself, where the flow is slow and thereby favorable for the durability of the heat transfer surfaces.
A portion of the solid particles which are first carried upwardly entrained with the gas, flows down along the riser walls, back to the lower section of the reactor. This portion is partly cooled provided that the wall is a cooling surface. Cooling of the solid particles may be further improved by providing the lower section of the wall with a pocket which collects the solid particles flowing down along the wall and then leads them to the lower section of the return duct, preferably to heat transfer surfaces. Thus, also a portion of those solid particles which the gas is not capable of carrying as tar as to the particle separators, is subjected to efficient heat transfer.
The method and apparatus according to the invention provide a simple arrangement for minimizing wear of heat transfer surfaces in the gas cooler. At the same time, power consumption is lowered compared to the prior art. Furthermore, in the arrangement according to the invention, the heat energy released by the gases is effectively utilized, e.g., by generated superheated steam.